1. Field of the Invention
The present invention is directed to a technique for computing the distance an observer may be from an object or source of illumination and still see that object or source. More specifically, this invention relates to apparatus for calculating visual range. Accordingly, the general objects of the present invention are to provide a novel and improved method and apparatus of such character.
2. Description of the Prior Art
The guidance of all transportation vehicles is intimately related to the ability of a human operator to clearly see the pathway ahead of the vehicle and maneuver the vehicle appropriately. As traffic volumes and speeds increase, there is a concommitant increase in the need for improved apparatus for accurately measuring visibility through the atmosphere. Such apparatus could find utility in assisting the guidance of land and naval vehicles as well as aircraft. Considering the environment of an aircraft, it is necessary to provide the pilot with information as to how far he can see runway lights and similar information must be provided to the ground controllers so that they will know when the local visibility conditions require the closing of a runway.
Techniques for computing runway visual range (RVR) and apparatus for use in the implementation of such techniques are presently known. However, all previously proposed or available apparatus have the limitation that they do not directly measure human visibility. Thus, prior art devices for use in visual range measurement; as exemplified by U.S. Pat. Nos. 3,510,225, 3,519,354, 3,745,350, 3,746,452 and 3,782,824; provide output information in the form of the atmospheric extinction coefficient. This parameter is not easily interpreted by human observers and thus is not particularly useful in informing a pilot or driver, respectively, as to how far he can see a certain visual clue such as, for example, landing lights or highway markers.
Many airports presently employ a system for assessing visibility which includes a runway visual range transmissometer. The system presently approved by the Federal Aviation Administration employs a field sensor, which includes a light source and receiver spaced 250 feet apart, placed adjacent to the runway. The field sensor measures the atmospheric transmittance, of the fixed 250 foot path. The atmospheric transmittance which may be expressed as the extinction coefficient of the atmosphere, is employed as an input to an expensive, large and immobile digital computer. The digital computer is programmed to provide an output which represents the distance that an average pilot can see runway edge lights under the prevailing atmospheric transmittance conditions. In accomplishing this objective the computer will solve two physchophysical equations in order to convert the extinction coefficient or atmospheric transmittance to visual range. The first of these equations is Allard's law which may be expressed as follows: ##EQU1## where: E.sub.t is the illuminance threshold; i.e., a property of the eye and the background lighting conditions;
I.sub.o is the luminous intensity of the specific target light (the runway edge lights, for example); PA1 t.sub.b is the atmospheric transmittance measured over a path length b (the distance between the light source and receiver of the field sensor); and PA1 R is the visual range. The visual range is also computed using Koshmieder's law which may be expressed as follows: EQU C.sub.R = C.sub.o (t.sub.b) .sup.R/b ( 2) PA1 C.sub.o is the contrast of a target; and PA1 C.sub.r is the observed contrast. The limiting value of the contrast threshold is taken to be 5.5% for aviation purposes. The digital computer solves both equations for R and selects the larger visual range value for display. In performing its function the digital computer in the present FAA transmissometer is essentially a loop-up table where pre-computed value pairs of t.sub.b and runway visual range are stored. The necessity of employing an expensive and essentially immobile digital computer to calculate, from input information in terms of the extinction coefficient of the atmosphere, the instantaneous visual range has precluded the widespread provision of visual range information to vehicle operators and, in the case of aircraft, has limited runway visual range determination to those airports having commercial service, to the detriment of the general aviation community.
where: